Energy harvesting techniques and systems are generally focused on renewable energy such as solar energy, wind energy, and wave action energy. Solar energy is conventionally harvested by arrays of solar cells, such as photovoltaic cells, that convert radiant energy to direct current (DC) power. Such radiant energy collection is limited in low-light conditions, such as at night or even during cloudy or overcast conditions. Conventional solar technologies are also limited with respect to the locations and orientations of installment. For example, conventional photovoltaic cells are installed such that the sunlight strikes the photovoltaic cells at specific angles such that the photovoltaic cells receive relatively direct incident radiation. Expensive and fragile optical concentrators and mirrors are conventionally used to redirect incident radiation to the photovoltaic cells to increase the efficiency and energy collection of the photovoltaic cells. Multi-spectral bandgap-engineered materials and cascaded lattice structures have also been incorporated into photovoltaic cells to improve efficiency, but these materials and structures may be expensive to fabricate. Multiple-reflection and etched-grating configurations have also been used to increase efficiency. Such configurations, however, may be complex and expensive to produce, and may also reduce the range of angles at which the solar energy can be absorbed by the photovoltaic cells.
Additionally, conventional photovoltaic cells are relatively large. As a result, the locations where the photovoltaic cells can be installed may be limited. As such, while providing some utility in harvesting energy from the electromagnetic radiation provided by the sun, current solar technologies are not yet developed to take full advantage of the potential electromagnetic energy available. Further, the apparatuses and systems used in capturing and converting solar energy are not particularly amenable to installation in numerous locations or situations.
Turning to another technology, frequency selective surfaces (FSSs) are used in a wide variety of applications, including radomes, dichroic surfaces, circuit analog absorbers, and meanderline polarizers. An FSS is a two-dimensional periodic array of metal elements to form an RLC circuit. For example, an FSS may include electromagnetic antenna elements. Such antenna elements may be in the form of, for example, conductive dipoles, loops, patches, slots or other antenna elements. An FSS structure generally includes a metallic grid of antenna elements deposited on a dielectric substrate. Each of the antenna elements within the grid defines a receiving unit cell.
An electromagnetic wave incident on the FSS structure will pass through, be reflected by, or be absorbed by the FSS structure. This behavior of the FSS structure generally depends on the electromagnetic characteristics of the antenna elements, which can act as small resonance elements. As a result, the FSS structure can be configured to perform as low-pass, high-pass, or dichroic filters. Thus, the antenna elements may be designed with different geometries and different materials to generate different spectral responses.
Conventionally, FSS structures have been successfully designed and implemented for use in radio frequency (RF) and microwave frequency applications. As previously discussed, there is a large amount of renewable electromagnetic radiation available that has been largely untapped as an energy source using currently available techniques. For instance, radiation in the ultraviolet (UV), visible, and infrared (IR) spectra are energy sources that show considerable potential. However, the scaling of existing FSS structures or other similar structures for use in harvesting such potential energy sources comes at the cost of reduced gain for given frequencies. For example, nano-scale resonant elements (also referred to as nanoantennas and nantennas) have experienced substantial impedance mismatch causing less than 1% power transfer, limiting the usefulness of such devices.
Scaling FSS structures or other transmitting or receptive structures for use with, for example, the IR or near-IR spectra also presents numerous challenges due to the fact that materials do not behave in the same manner at the nano-scale as they do at scales that enable such structures to operate in, for example, the radio frequency (RF) spectrum. For example, materials that behave homogeneously at scales associated with the RF spectrum often behave non-homogeneously at scales associated with the IR or near-IR spectra.